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    Scope position relative to frame
    From: Bill B
    Date: 2007 Jan 17, 21:07 -0500

    I've not had the time and sun simultaneously to do extensive checks, but
    initial results of three rounds of IE checks have raised questions in my
    mind.
    
    During Frank's laser IE check I attempted to center the horizon glass/mirror
    intersection in the scope, and shoot the laser down the center of the scope.
    
    Earlier this week I was doing IE checks with the sun with the scope mounted
    in the same position as the laser tests.  To my surprise the IE had shifted
    0!3 towards off the arc from its usual. (On the arc figure reduced by 0!3.)
    4SD was 0!1 over target in all three rounds, and n-1 sigma (s) was less that
    0!1 in all sets.
    
    I then moved the scope to the position closest to the frame (usual position)
    and did more sun IE checks.  That brought the IE back to the usual figure.
    
    I understand why it is important to center the images vertically in the
    scope (ideally the center of the scope will coincide with the center of the
    foreshortened surface of the front-silvered index mirror when at the highest
    angular setting on the arc).
    
    I am at a loss as to why a sun IE check should vary depending on scope
    distance from the frame.  Only two things immediately come to mind.
    1. Distortion caused by scope optics when off center
    .
    2. Collimation error caused by an less-than perfect scope mounting system.
    
    Two initial questions:
    
    1. What are the symptoms/affects of collimation error?
    
    2. Can anyone suggest why this is happening, other than operator error?
    
    Any input would be appreciated.
    
    Thanks
    
    Bill
    
    
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